The invention relates to a method for eliminating the background brightness variation of a video signal, as it is known for example from the British patent specification No. 1295773.
For a video signal V at the moment t it applies that EQU V(t)=a(t)+s(t)+r(t)
in which
a(t)=the background brightness signal, a signal which, due to properties of the camera and differences in exposure, slowly varies, and which has the property a(t).apprxeq.a(t-T), in which T=the scanning time for one line (64 .mu.s);
s(t)=the signal which is caused for example by information signals of a document, and which is quickly varying;
r(t)=a signal relating to camera and paper noise.
Said three partial signals are independent of one another. The signal s(t) is the desired signal, whereas the signals a(t) and r(t) are disturbing elements, which have to be eliminated.
In the methods known so far the highest voltage level that occurs is detected in a circuit having a specified time constant. By means of the signal thus obtained a threshold voltage can be formed, which tries to follow the course of the background level in the best possible way. When choosing the time constant a compromise has to be found. In the case of a small time constant the last level observed is maintained insufficiently long and long-duration pulses cannot be detected, whereas in the case of a large time constant the level will not be followed properly. As regards the difference in voltage between the threshold signal and the video signal, a constant difference can be chosen, small enough to detect even pulses with a small amplitude, but just great enough to be free from any inconvenience caused by the signal factor r(t).
The situation described above is linked up with the fact that in the known methods, use is made only for a small length of the line that is being scanned at a certain moment.
The object of the invention is to improve considerably the methods known so far.